Ultrasonic sensors used for measuring the presence of a liquid in a vessel are well known. One type is called a contact, or invasive, type sensor. This type of sensor utilizes a support structure defining a gap into which the liquid flows. In such sensors a piezoelectric (crystal) transducer element is bonded by a suitable adhesive, such as an epoxy, to the inner face of one or both surfaces of the structure forming the gap. The element transmits energy and if there is liquid present in the gap the energy will travel across the gap. If there is no liquid in the gap the energy will be attenuated in the air.
In an invasive sensor using one element, energy is transmitted through the liquid to the opposing surface of the gap structure from which it is reflected back to the element. Detection of the reflected signal indicates the presence of liquid in the gap. In a two element invasive type sensor, there is an element on the inner face of each surface of the gap defining structure. Here, the second element receives the energy transmitted through the liquid across the gap and indicates liquid presence. The same effect occurs when there is any other type of sound energy transmitting material in the gap.
Another well known sensor is the non-contact, or non-invasive, type in which the sensor is mounted externally of a pipe or vessel. In this type of sensor the liquid does not come into contact with the sensor. The transmitted energy from the element is travels through the wall of the pipe or vessel to which it is mounted and the liquid to be reflected from an opposing wall of the pipe or vessel to which the sensor is mounted and is reflected back to the element. If there is no liquid, the energy is attenuated in the interior of the pipe or vessel. In a properly operating sensor a portion of the energy is reflected back to the element from the interface of the interior of the wall of the pipe or vessel and its interior, whether wet or dry.
It is desired to provide such sensors with self-test capability to check, either continuously or on demand, failure of any of the sensor components. This includes the bonding of the piezoelectric element, or elements, to the respective inner face, or faces, of the gap surfaces. That is, it is desired to determine if an element has become dis-bonded. If an element has become dis-bonded, the sensor will always signal a `dry` condition, that is, that there is no liquid in the gap. This indication will be given falsely even if the gap is `wet`, that is, there is liquid present. Dis-bonding of an element occurs for a variety of reasons such as dropping of the sensor, rapid temperature changes (thermal shock) and other factors. In effect, dis-bonding of an element causes the sensor to become inoperative. A similar situation occurs in non-contact type sensors.
One arrangement intended to check the integrity of a sensor is disclosed in U.S. Pat. No. 5,269,188 to Esin et al granted Dec. 14, 1993. This patent discloses a frame type sensor support structure in which a stem connects the two gap forming surfaces. In a sensor using two elements, one is bonded to the interior of one of the surfaces forming the gap and serves as a transmitter and the other is bonded to the other surface and serves as a receiver. When energy is transmitted from the transmitter element an amount passes, or leaks, directly through the stem part of the sensor support frame connecting the two surfaces forming the gap and is received by the receiver element for use as a self-test signal. This leaked energy does not travel across the gap and it is used as a self-test signal.
A period of time, or time window, for this self-test signal travelling through the support frame to be received by the element mounted to the other surface of the frame gap is set in relation to a time window at which the main energy passing through liquid in the gap, if present, is to be detected. The appearance of a signal in the self-test time window, which should occur both in wet and dry gap conditions, is to indicate that the sensor is supposed to be functioning properly. However, this arrangement is not fully capable of checking for dis-bonding of a piezoelectric element, or elements. This is due to the fact that energy can leak out from the back surface or sides of a dis-bonded element and travel through the stem to the receiver element. This situation is sometimes referred to as `cross-talk`. Cross-talk also can occur in a sensor using only one element in that the part of the energy used for self-test can leak through the stem to the opposing surface for transmission through liquid in the gap back to the one element and give an indication of the sensor operating properly or not.